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VOTE YES. Supporters hold placards as they attend a same-sex marriage rally in Sydney on September 10, 2017. Photo by Saeed Khan/AFP

SYDNEY, Australia – Thousands of Australians dressed in rainbow colours rallied Sunday, September 10, in support of same-sex marriage ahead of a postal ballot, as polls showed the "yes" campaign's lead shrinking despite backing from the nation's top political leaders.

Carrying vibrant posters and calling for marriage equality, the marchers packed the streets of central Sydney and Brisbane two days before the ballot papers were set to be mailed out to some 15 millions Australians.

Marriage equality has attracted popular support in recent years, but such unions are yet to be legalized in the country amid political wrangling. In a bid to resolve the issue, the conservative government moved to hold a postal ballot after its preferred option of a national referendum was twice rejected by the upper house.

"If a majority votes yes, then we will ensure a... bill is presented to the parliament which will legalize same-sex marriage," Prime Minister Malcolm Turnbull on Sunday told those conservative Liberal and National politicians who support the "yes" campaign.

"Our expectation is that should be accomplished by the end of the year -- it will sail through the parliament."

But the success of the "yes" vote is not certain, with a poll conducted for a major same-sex marriage advocacy group and shared with Fairfax Media on Saturday, September 9, showing that support for the change was falling.

The poll, conducted between late August and early September, showed that 58.4% of those surveyed supported a "yes" vote, down 6 percentage points from just two weeks earlier.

Support for the "no" vote increased by two percentage points to 31.4%, while the "unsure" vote rose by 3 percentage points to 10.2%.

The survey also showed that turnout for the voluntary ballot could be very low, and at just 58% of those aged 18-34 -- the age group viewed as more receptive to gay marriage.

Labor opposition leader Bill Shorten, who supports the change, told the Sydney rally the ballot was about "our identity as a nation".

"Do we believe in equal care for all, equal rights for all?... We can win this thing. We can climb this mountain."

Turnbull, a political moderate who supports marriage equality, is opposed by some in his own Liberal-National governing coalition.

"No" campaign supporters, including hundreds who rallied in Sydney on Saturday, argue that changing marriage laws would infringe on religious freedom and children's rights.

Andrew Pasco, one of thousands of people who packed the streets of Sydney for the "yes" campaign rally on Sunday, said he was worried the polarising debate could discourage moderate Australians from voting.

"I am genuinely concerned that as the conversations get driven to extremities on both the left and right, that that will also alienate the people in the middle that aren't directly impacted to speak up and have a voice," he told Agence France-Presse.

The postal vote will close on November 7, with the result known later that month. – Rappler.com

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Welcome to Rappler, a social news network where stories inspire community engagement and digitally fuelled actions for social change. Rappler comes from the root words "rap" (to discuss) + "ripple" (to make waves).